Sound reproduction



May 16, 1939.

R. S. MORSE SOUND REPRODUCTION Filed Sept. 30, 1957 R/bh 0rd 5 Morse INVENTOR AT T ORNEYS Patented May 16, 1939 UNITED STATES PATENT OFF r ce SOUND REPRODUCTION Richard S. Morse, Rochester, Eastman Kodak Company,

Y., assignor to Rochester, N. Y., a

corporation of New Jersey Application September 30, 1937, Serial'No. 166,525

' 4 Claims. (o1.179 '100.s)'

This invention relates to sound reproduction and particularly to electric circuits used for the reproduction of sound from sound-on-film records, such as used with motion pictures.

It is an object of the invention to provide a sound reproducing system having high fidelity over a greater range of sound volume than hitherto available.

Ordinarily, the volume range of sound is limited at the lower end by extraneous noise due to various causes and at the higher end by the capacity (i. e. 100 per cent modulation) of the recording device employed. Between such limits there is a range of about 40 to 50 decibels which is, however, inadequate for many types of orchestral renditions. This range may be extended by using some system of volume compression during recording and then to compensate for this compression, a system of volume expansion during reproduction.

It is a particular object of the invention to provide a simple method of volume expansion during reproduction and more specifically one which can be readily employed with systems wherein an exciter lamp is operated from a high frequency oscillator.

According to the invention, a portion of the photoelectric cell output carrying the signal being reproduced, is sidetracked before reaching the loud speaker and after amplification and rectification presents a D. C. potential which is a function of the amplitude of the signal (not necessarily linear). This D. C. potential is introduced to the grid of a high frequency oscillator tube and regulates the output thereof,

which supplies power to the exciter lamp illuminating the photoelectric cell through the sound record. Thus any increase in signal volume, increases the oscillator regulating potential which in turn increases the oscillator output and the brightness of the exciter lamp. This causes an increase in the sensitivity of the exciter lampphotocell combination which results in an increase of the photocell output. Thus the de- 5 sired automatic volume expansion is obtained which is opposite in effect to the usual automatic volume control systems used in radio receiving sets wherein approximately constant 50 output is sought.

Other objects and advantages of the present invention will be apparent from the following 3 description of one embodiment of it when read in connection with the accompanying drawing which 55 Shows:

'An electric circuit for sound reproduction-incorporating the invention. I

In this drawing an exciter lamp l illuminates the sound record 2 on a film 3 and the light transmitted thereby falls on a photoelectric cell 4. The output of this photoelectric cell is thus modulated in the usual way by the sound track 2 and carries the signal to be reproduced. This signal is passed through an amplifier 5 and operates a loud speaker 6 through its accompanying circuit 1.

According to this invention a portion of this output is passed through a control 8, a condenser 9, an amplifier shown as a triode l0, another condenser ll and a suitable rectifier [2 shown as a diode to present at the point l5 in an oscillator circuit M, a potential which varies in accordance with the amplitude of the output of the photoelectric cell 4. The absolute value of this potential may be controlled by a variable resistance l3. This potential on the point It may be introduced to the oscillator M in any manner whereby the output thereof is controlled thereby. In the specific arrangement shown an oscillator tube I6 having a coupled grid coil I! and plate coil I8 is arranged so that the point I5 is connected directly to the control grid of the tube I 6.

The output of the oscillator I4 is passed through a coil l9, inductively coupled to the oscillator coils I1 and I8, and through leads 20 to operate the exciter lamp l. Thus, any increase in amplitude of the signal on the sound record 2 is recorded as an increase in signal output of the photoelectric cell 4 which, in turn, increases the potential at the point l5 of the oscillator M. This increase in potential at the point [5 increases the output of the oscillator, which output is carried by the coil l9 and leads to the exciter lamp. 7

In cases where the soundtrack 2 has been recorded linearly up to some predetermined amplitude and above this point volume compression has been used, it is desirable to incorporate in the invention means for determining the point (in amplitude) at which volume expansion begins. For this reason, I prefer to include some form of time delay circuit many of which are well known; for example, a simple potentiometer circuit 2| may be included in the cathode circuit of the rectifier tube l2. On the other hand, this potentiometer 2| may be entirely omitted by connecting this circuit directly as shown by the broken line 22.

Having thus described in detail one embodiment of my invention, I wish to point out that it is not limited to the specific arrangement shown, but is of the scope of the appended claims.

What I claim and wish to secure by Letters Patent of the United States is:

1. An electric circuit for reproduction of soundon-film comprising an exciter lamp and photocell combination adapted to transform a soundon-film record into electrical modulation, a high frequency oscillator supplying power to the ex-.

lator connected to the lamp, means for controlling the output of said oscillator and rectifying means across the input of the loud speaker for operating said control means.

3. In a sound-on-i'ilm reproduction system adapted to operate a loud speaker, an exciter lamp circuit comprising a high frequency oscillator connected to the lamp, means for controlling the output of said oscillator, rectifying means across the input of the loud speaker for operating said control means and means cooperating with the rectifying means for delaying its operation until the amplitude of the signal in said input reaches a predetermined value.

4. In a sound on film reproduction system adapted to operate a loud speaker, an exciter lamp circuit comprising a high frequency oscillator for energizing the lamp, means for controlling the output of said oscillator and means across the input of the loud speaker for operating said control means.

RICHARD S. MORSE. 

